This quote, taken from the beginning of today’s first reading (Col 1:24-2:3), is the cause of much consternation, confusion, and debate among Christians. Was not Jesus’ suffering (more than) sufficient for the redemption of the world? What could I possibly add?
The Ignatius Catholic Study Bible is helpful:
“These words could be misconstrued to mean that the suffering of Christ was not sufficient for redemption and that the suffering of the saints must be added to complete it. This, however, would be heretical. Christ and the Church are one mystical person, and while the merits of Christ, the head, are infinite, the saints acquire merit in a limited degree. What is ‘lacking,’ then, pertains to the afflictions of the entire Church, to which Paul adds his own amount (St. Thomas Aquinas, Commentary on Colossians 1, 6).” (Ignatius Catholic Study Bible, The Letters of St. Paul to the Philippians, Colossians, and Philemon, 30)
In the same section, it states that “[s]uffering is a mission for all the faithful as a means of conforming ourselves to Christ.” This goes a long way in making sense of suffering. St. Alphonsus Laiguori helps us here:
“‘[F]or the merits of the Passion to be applied to us according to St. Thomas (Summa theologiae, III, q. 49, a. 3), we need to cooperate (subjective redemption) by patiently bearing the trials God sends us, so as to become like our head, Christ” (St. Alphonsus, Thoughts on the Passion, 10).” ( The Navarre Bible, Captivity Epistles, 171)